The present invention relates generally to power supply circuits which generate direct current (DC) power from alternating current (AC) power and, more particularly, to a power factor correction circuit for an AC to DC power supply.
Power supplies for providing direct current (DC) power from an alternating current (AC) power supply are well known and used to power loads ranging from lighting to personal computers. Such power supplies typically include a rectifier circuit to charge a large capacity charge storage capacitor which supplies the DC power. Current input to these circuits tends to be in short pulses which have high peaks and are of short duration when compared to the substantially sinusoidal waveform of the input AC power supply. Unfortunately, the pulsing input current is rich in odd harmonics and results in a poor power factor at the input of the power supply.
A large variety of arrangements have been developed to overcome the problems associated with pulsing input current caused by rectification and energy storage in power supply circuits. Many of these arrangements are active and include transistorized control circuitry either in a discrete or integrated form. Typically, these circuits monitor the input current and use various control functions, such as pulse width modulation (PWM), to shape the input current to substantially match the input voltage and thereby produce a nearly 100% power factor for a power supply circuit.
Known active control arrangements suffer from two principal problems. First, these arrangements may be sensitive to line transients and accordingly, measures must be taken to protect the active circuitry. Second, to be able to use reasonable size power components, high frequency operation is required and can generate noise and produce line isolation problems. Correction of noise and line isolation problems requires additional circuitry which adds to the complexity and costs of these arrangements.
Many passive arrangements also exist. Although the passive arrangements are not capable of delivering substantially 100% power factor, they can provide measurable improvement in the power factor. All presently known passive arrangements depend on linear behavior of inductive components. Unfortunately, to achieve both linearity and the needed inductances for high power applications at typical low AC supply frequencies, the required inductors are physically very large making them undesirable for many applications which require compact packaging or some amount of portability.
Linear behavior of known passive arrangements fall generally into two classes of operation. The first is resonant circuit operation wherein an inductor and a capacitor are selected to resonate at preselected frequencies which are multiples of the AC power supply frequency. The inductor can be connected in series or parallel with the AC power supply and load, and is described in terms of linear circuit elements and phasor analysis in view of its linear operation and sinusoidal nature of operation.
The principal limitations of resonant linear arrangements are: poor control of the corrected wave shape due to the nature of large steel inductors and an associated low Q factor of the tuned network; and, the large size of the inductors due to the need of linear inductors to pass large currents and still maintain linear operation and low I.sup.2 R losses.
The second class of passive operation is bandpass limiting. As is well known in the art, any wave shape other than a pure sinusoid is made up of other harmonics of different phases and magnitudes. The pulsing current wave created by rectification and filtering of the AC input supply is a complex wave of this nature. A bandpass limiting arrangement controls the bandpass of power from the AC power supply to block out the higher order harmonics and thereby prevent the distortion of the line current from taking place. The principal limitations of bandpass limiting are: large linear inductors are required; there is a large reduction in the effective line voltage; and, several stages of filters may be required to accomplish the desired bandpass limiting.
Accordingly, there is a need for a simple, inexpensive power phase correction circuit for an AC to DC power supply which provides a substantially improved power factor. Preferably, the power phase correction circuit would be passive for ultimate simplicity and reduced expense.